In digital reproduction of documents such as in the digital copier environment, a document is first optically scanned and converted to a gray scale image. In the case of color reproduction, the document may be converted to a gray scale image of several separations, such as the R, G and B separations.
In order to produce a hard copy of the scanned and digitized image, the image has to be further processed according to the requirements of the marking engine. For example, if the marking engine is capable of bi-level printing, then the image has to be rendered into a 1-bit bit map for printing. To preserve the appearance of a gray scale image in a binary output, often some digital halftoning process is used in which the multi-bit input image is screened with a periodic array. However, if the original image itself contains halftone screen, objectionable moire patterns may occur due to the interference between the original and the new screens. Also, while dot screen halftoning may be good for rendering continuous tone originals, it may degrade the quality of text and line drawings. Often a document contains different types of images.
In order to achieve optimal image quality in document reproduction, a system capable of automatically identifying different types of images on or within a scanned original image is needed. For example, if an image part is identified as halftone, then some kind of low-pass filtering may be applied prior to halftone screening so the gray scale appearance can be preserved without introducing moire patterns. For text area, some sharpness enhancement filter could be applied and other rendering techniques such as thresholding or error diffusion could be used.
In classifying images or portions of images for the most suitable processing for subsequent printing or other purposes, one specific source of error is the misclassification of other types of images as “low frequency halftones.” For example, some pixels in continuous-tone (contone) area could be falsely detected as peaks or valleys due to noisiness or other reasons. In high-frequency halftone area, some peaks or valleys are not detected, due to the weakness of the signal or other reasons. These areas will appear to be of halftone dot frequency (peak and valley count over a length within the image) which falls into the range of low-frequency halftones. A contone or high-frequency halftone image is liable to be classified incorrectly as a low-frequency image, and be processed accordingly, and vise versa. The consequences of misclassification of image data could include inefficiency in processing the image data, and undesirable artifacts in any subsequent printed image, such as a digital copy.